204 PROGRESS OF SCIENCE. 



than the moon necessarily keeps this hemisphere facing us. 

 Lagrange also explained that, owing to the balancing 

 movement called ' ' LIBRATION " to which the moon is 

 subjected by going alternately a little faster or a little slower, 

 it must show us a thin edge of the face which is hidden from 

 us a glimpse on either side in turn so that four-sevenths of 

 the moon's surface can be seen instead of one-half only. 

 And owing to the Iteration, too, the moon's apparent 

 diameter varies from a maximum value of 33' 31*1" to a 

 minimum value of 29' 21 "9". Lagrange further solved a 

 -question of much vaster importance, viz. : that of the 

 stability of the orbits of the planets (1776). Uniting his 

 own with Laplace's investigations, together with all the facts 

 known on the subject, in one grand mathematical problem, 

 he formulated the generalisation, that, whatever the infinite 

 changes, perturbations, and variations affecting planets, yet, 

 in the course of ages, every part of the system remains 

 absolutely stable, and subjected to the force of gravitation 

 which keeps them eternally moving round the sun ; so that 

 he demonstrated, as we said at starting, the theory of the 

 Invariability of the great axes. Laplace, himself a mathe- 

 matical genius of the first order, justly said, in admiration 

 of Lagrange's achievements, that " of all the inventors who 

 had contributed most to the advancement of human know- 

 ledge, Newton and Lagrange appeared to possess, in the 

 highest degree, the happy tact of distinguishing general 

 principles among a multitude of objects enveloping them, 

 and this tact he conceived it to be the true characteristic of 

 scientific genius." We may add that Lagrange considered 

 less astronomical questions than the mathematical analysis to 

 which they might give rise. 



17381822. Herschel (William), justly called the CO- 

 LUMBUS OF THE HEAVENS, was indeed the greatest of all 

 observers, and one of the founders of astronomical physics be- 

 sides. With a telescope forty feet long, of his own making, 

 he discovered and mapped nearly 2,500 NEBULA in the 

 Northern Hemisphere, and also numerous double stars, only 

 a few of which were known before his time. He detected 

 over 500 DOUBLE STARS disseminated in the heavens, after 



